Measuring line-of-sight dependent Fourier-space clustering using FFTs

Davide Bianchi, Héctor Gil-Marín, Rossana Ruggeri, Will J. Percival

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Observed galaxy clustering exhibits local transverse statistical isotropy around the line-of-sight (LOS). The variation of the LOS across a galaxy survey complicates the measurement of the observed clustering as a function of the angle to the LOS, as Fast Fourier Transforms (FFTs) based on cartesian grids, cannot individually allow for this. Recent advances in methodology for calculating LOS-dependent clustering in Fourier space include the realisation that power spectrum LOS-dependent moments can be constructed from sums over galaxies, based on approximating the LOS to each pair of galaxies by the LOS to one of them. We show that we can implement this method using multiple FFTs, each measuring the LOS-weighted clustering along different axes. The N log(N) nature of FFTs means that the computational speed-up is a factor of >1000 compared with summing over galaxies. This development should be beneficial for future projects such as DESI and Euclid which will provide an order of magnitude more galaxies than current surveys.
Original languageEnglish
Pages (from-to)L11-L15
JournalMonthly Notices of the Royal Astronomical Society: Letters
Issue number1
Early online date4 Aug 2015
Publication statusPublished - 11 Oct 2015


  • astro-ph.CO
  • RCUK
  • STFC
  • ST/K0090X/1 and ST/K00283X/1


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